The broad objectives of this study will be to characterize basic interactions between parathyroid hormone and biological membranes, to develop a radioreceptor assay for parathyroid hormone, and to define the pattern of urinary excretion of cyclic AMP in patients with hypercalcemia, pseudohypoparathyroidism, and hypomagnesemia. Plasma membranes from renal cortical tissue will be prepared and modified with specific hydrolytic enzymes and chemical probes. The effects of these agents will be tested on binding and biological responsiveness to parathyroid hormone and on the composition of protein and glycoprotein components of the membrane. These interactions will be used as a basis for assaying biologically active parathyroid hormone in plasma and defining the nature of the receptor defect in pseudohypoparathyroidism. Renal clearance of cyclic AMP will be tested as a useful technique for diagnosis and followup of patients with disorders of calcium metabolism and correlated with results of the radioreceptor assay on plasma from these patients.